This invention relates to semi-submersible offshore vessels, and more particularly to production facilities suitable for development of oil and gas mineral reserves in water depths of one thousand to six thousand feet, or greater depths.
Semi-submersible vessels are widely used for drilling and production operations in offshore locations for development of mineral subsea resources. These semi-submersible vessels provide relatively easy mobility and can be deployed near a prepared well site and then anchored by catenary or semi-taut mooring lines.
Semi-submersible platforms usually comprise of horizontal buoyant members (or pontoons) submerged below the water surface and supporting production or drilling platforms by columns extending from the underwater pontoon to a level above expected wave action. The pontoons are located below the expected height of wave action to reduce the wave-induced response of the platform. Semi-submersible production platforms are usually deployed after exploratory operations have been completed and the nature of mineral deposits and exact locations have been identified.
Construction and outfitting of a production platform has been and remains extremely costly, requiring several years of construction and preparation. Once completely outfitted, the production platform is usually brought to the well site, moored, and set for production operations by connecting the flow lines and the export pipe lines to the equipment on the platform.
The pontoons utilized for semi-submersible vessels may be designed as separate horizontal members or as ring pontoons. This invention relates to a semi-submersible vessel utilizing a ring pontoon, which supports vertical columns. The columns support a superstructure deck or decks. The pontoon of the instant invention has adjustable ballast capability to allow the vessel to be easily transported to the production location and, after reaching the desired location get ballasted to cause the pontoons to become submerged below the surface of the water and provide the necessary stability to the vessel.
It is an object of the present invention to provide a semi-submersible floating production facility that can be completely outfitted at the dry dock where the hull is constructed prior to the offshore installation.
It is another object of the present invention to provide a semi-submersible vessel utilizing a ring pontoon with a sufficiently large water plane inertia to ensure adequate stability while minimizing the vessel motion response. It is a further object of the present invention to provide a semi-submersible vessel with an open frame deck that allows changing of production modules from oil to gas production in an easy and inexpensive manner.
It is still a further object of the present invention to provide a semi-submersible vessel that can be moored and does not require dynamic positioning equipment.
These and other objects of the present invention are achieved through a provision of a semi-submersible floating production vessel with a ring pontoon generally shaped as an equilateral triangle. Three main columns extend from three corners of the triangle, the main columns contributing the significant portion to the water planearea of the vessel. A plurality of thin secondary supporting columns extends from the ring pontoon upwardly to support an open frame deck. The secondary columns extend from approximately geometric centers of the ring pontoon connecting members between the main columns.
The open frame deck structure facilitates modular construction and allows positioning of production modules on the deck of the vessel and changing of the modules from oil- to gas-adapted production modules in a relatively inexpensive, expeditious manner. Special liquid storage tanks, such as methanol tanks are supported below the deck. The deck also supports dual fuel electrical power generators as part of the modular assembly.
The vessel has a plurality of production and export risers that are secured to the vessel below the water line. The vessel is adapted for semi-permanent mooring with pre-tensioned mooring lines that are attached to the vessel by swivel padeyes secured below the water line. Such arrangement allows transfer of the vertical component of the load from the mooring lines to the vessel main columns. Additionally, the vessel does not require a dynamic positioning system with associated thrusters, diesel generators and control systems.
The ring pontoon is divided into a plurality of separate ballast compartments. A compressed air ballast system is utilized to selectively empty each ballast compartment for inspection and repair, if necessary. As a result, the need for a separate pump room and associated equipment is eliminated.
The vessel structure extensively uses box girders and flat plate girders for ease of construction and maintenance. This design significantly reduces the time needed for construction of the vessel, reduces the steel requirements and increases allowable deck loads. The vessel design more effectively dampens the heave motions, reduces the roll motions and stresses in the primary structural members, as well as improves the fatigue life.